The quest to develop larger and larger semiconductors of the dynamic random access memory (DRAM) type is a well-known goal. The industry has steadily progressed from DRAMs of the 16K type, shown in U.S. Pat. No. 4,081,701 issued to White, McAdams and Redwine, and the 64K type, shown in U.S. Pat. No. 4,055,444 issued to Rao, to DRAMs of the 1M type, as described in U.S. Pat. No. 4,658,377 issued to McElroy. DRAMs of the 4M type are now being produced. Production plans for high density 16M DRAMs of submicron technology now exist and experimentation of 64M DRAMs has begun. One factor furthering the development of larger DRAMs is the reduction in memory cell geometries as illustrated in U.S. Pat. No. 4,240,092 to Kuo (a planar capacitor cell), and as illustrated in U.S. Pat. No. 4,721,987 to Baglee et al, (a trench capacitor cell).
Manufacturers of dynamic memories are concerned with data retention time characteristics. A test referred to as "pause-refresh" measures the time retention of the memory. This test delays the normal refresh cycle time. Data bits initially stored in the memory cells are measured as the refresh cycle time is extended. The retention time of the memory device is thus given by the longest length of time the refresh cycle may be extended before the bits stored in memory begin to fail.
A difficulty in fabricating high density integrated devices such as dynamic memories is that memory cells around edges of the memory array, edge cells, typically have worse pause refresh characteristics than memory cells central to the memory array. Edge cells begin to lose data sooner than random center cells of a memory array. If the edge cells do not have sufficient pause refresh capabilities, then the memory device is suspect.
It is the principal object of this invention to provide a semiconductor memory device that is less susceptible to edge cell failures. Other objects and advantages of this invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings.